1. Technical Field
The present disclosure relates to a source driver for a flat panel display and, more particularly, to an output buffer with an improved output deviation and a source driver for a flat panel display having the output buffer.
2. Discussion of Related Art
Typical liquid crystal displays (LCDs) which are comprised of a liquid crystal layer that is disposed between two substrates, adjust the transmissivity of light through the liquid crystal layer by adjusting a voltage applied to the liquid crystal layer and, thus, display a desired image. Examples of such typical LCDs include thin film transistor (TFT)-LCDs using TFTs as switching devices for switching a liquid crystal layer. One of the biggest advantages of polysilicon TFT-LCDs, which are a type of TFT-LCDs is that circuitry consisting of a plurality of MOS transistors formed on a glass substrate can be embedded in polysilicon TFT-LCDs. Currently, a plurality of drivers can be integrated on a glass substrate using a low-temperature polycrystalline silicon (LTPS) process, and an entire system including a controller can be integrated on a glass substrate using a system-on-glass (SOG) method.
TFT-LCDs include a source driver for driving a plurality of liquid crystal cells arranged on a liquid crystal panel. A source driver includes a plurality of output buffers that provide a gray voltage signal to a liquid crystal panel, and each of the output buffers comprises an operational amplifier that is comprised of transistors. In a case where an operational amplifier of a source driver is comprised of transistors, output voltages from the output buffers corresponding to the same input data may have a large deviation. Such deviation is referred to as an offset voltage. An offset voltage varies considerably according to the characteristics of the transistors that produce the offset voltage. Therefore, even if the same data voltage is applied to a source driver, gray voltages applied to a liquid crystal panel in response to the same data voltage may not be identical due to the deviation in output signals from an output buffer of the source driver. When a liquid crystal panel is provided with different gray data for the same input data voltage by a source driver, the liquid crystal panel may malfunction or distort signals, thus deteriorating the quality of images.
A deviation in the output of an output buffer of a source driver is generated due to a deviation in the threshold voltage of a transistor. The threshold voltage of a transistor is a voltage needed to turn on or off the transistor and can be defined as a function of the thickness of a gate insulation layer interposed between a gate electrode and a channel region and the doping concentration of the channel region. The thickness of the gate insulation layer and the doping concentration of the channel region can be altered by process deviations that occur in the manufacturing operation. Conventionally, transistors of an output buffer are manufactured to be identical to one another. Although the gate insulation layers of the transistors are attempted to be formed to have the same thickness this is not always possible during manufacturing, thereby causing a deviation in the output of the output buffer according to the threshold voltage of the transistors.
In order to address this problem, a method involving additionally installing an offset compensation circuit in an output buffer has been suggested. This method, however, increases the size of an output buffer, and this is undesirable.